When you look at fuel efficiency, the measure is nautical miles per gallon (NMPG) or the metric equivalent. In boat like all of ours, there is no such thing as a max efficiency point, i.e there is no maximum inflection point. As soon as you leave the dock, your NMPG does nothing but drop the faster you go. I believe it is always true that the faster you go, the lower your NMPG, and the slower you go, the higher your NMPG. TDunn's graph shows why.

So when we talk about "sweet spots" for cruising, that isn't any sort of maximum point for NMPG, because that maximum occurs at your boat's slowest speed. The "sweet spot" is what each of us feels is the best tradeoff between a speed that actually gets us to our destination, fuel consumption, and comfort onboard.

I appreciate what twistedtree and Psneeld both point out. As someone with a familiarity with physics, a long time boater, and general idiot, I guess in my mind when I think of the "sweet spot" I try to imagine that point where returns start to diminish greatly with increased power. I know that NMPG is not a linear function at any power range, but for most boats isn't there is a point where that curve starts to become significantly steeper? It is that point that I try to find with my boat. Then it is a matter of factoring in other issues like how soon my wife wants to get where we are going.

A knot below hull speed settles the boat in the trough of the wave not significantly climbing the the bow wave. One trades the extra power required to significantly climb the bow wave for riding a bit on the following wave getting a little push. Go hull speed and you loose the push of the following wave and gain the monsterous job of going "uphill" on the bow wave. Approximately one knot below hull speed is the "sweet spot" but that expression is usually directed to minimal vibration. But for this thread it means at the end of the relatively flat curve of power v/s speed that ramps up getting really steep about 1/2 a knot below hull speed.

Thanks, that was quite insightful and easy to digest. All of my boats have up to now planed unless they were dragging the anchor .

I know that NMPG is not a linear function at any power range, but for most boats isn't there is a point where that curve starts to become significantly steeper? It is that point that I try to find with my boat. Then it is a matter of factoring in other issues like how soon my wife wants to get where we are going.

In our boat, there are two points where the curve gets dramatically steeper. One is between "calculated" hull speed and getting onto plane, and the other is at the high-ish end of the RPM curve.

I simply looked at the nominal fuel consumption curves for our engine -- i.e., at various RPM increments -- and then did some real-world speed "tests" (comparisons of RPMs vs. speed). Made a table.

Seasoned that with a little reality (wind and/or current), made my best guesses about where my speed(RPMs)/fuel trade-offs would be.

Seasoned a little more by weather conditions and how fast we want to get some particular place on any given trip.

Very little science involved. Well, at least all I did was some simple math mixed with some empirical data.

The resistance curve of all boats is not necessarily an ever steeper vertical line. It can vary all over the place. Below is a graph of general resistance characteristics for various hull forms. The "round bilge planing hull" curve would be representative of a typical semi-displacement hull. Note the planing hull climbs a steep hill and then drops into a valley where resistance drops as speed increases. Also note that all of these curves are going in different directions below S/L = 1.0......there are a lot of variables.

I'd call it a semi-planing hull. Sharp entry and fairly flat aft section with a 3/4 length keel. It's designed to allow the 200+ hp models to plane and scoot along at 20-23 Kts. With my twin 85HP Perkins, I run around at displacement speeds.

Thanks for the compliment. She does look quite different than most other boats around here and fits into our lives quite comfortably.

You can get an estimate of fuel burn from horsepower fomulas. Pretty much every horsepower formula calcyulates horsepower from a combination of waterline length, displacement and speed. For a displacement hull or a semi-displacement hull running at displacement speeds, I think the Wyman formula gives the most reasonable results. That formula is:

Wyman: SHP=(Disp/1000)*(KTS/(LWL1/2))3

Where SHP is shaft horsepower, Disp is displacement in pounds and LWL is the waterline length in feet. For example a Grand Banks 36 (Disp = 26,000 lbs, LWL = 35.17') running at 8 knots requires about 64 horsepower. Similarly at 7 knots the power requirement is about 43 horsepower.

Similarly, a CHB 34 would need about 35 hp to go 7 knots and 52 hp to make 8 knots.

Diesel engines produce about 18 horsepower per gallon per hour of fuel consumption. So fuel consumption for the two boats listes above is:

Note that these numbers are approximations only since specific fuel consumption for diesels varies a bit (say 16-20 hp per gallon per hour) and the horsepower calculation is only a first approximation for flat water only. However these calculations give you a an idea of what to expect. They also illustrate how fuel consumption increases with speed. Note that going from 6 to 8 knots for these two boats more than doubles fuel consumption.

Tad, thanks for the graph. However, if you throw out the hydrofoil and hovercraft curves, all the curves have monotonic increases in resistance (= power requirement = fuel consumption) up to about S/L ratios of 2.5. The details differ, but the general form is the same. For typical "trawler type" boats S/L of 2.5 is in the range of 13-15 knots (30'-36' LWL), which is pretty fast for this type of boat. What happens at higher speeds is irrelevant to the go slow crowd.

Consider a Grand Banks 36. With a single Lehman 120 it is limited to a top speed of 10-11 knots and with twin 120 Lehmans might make 12.5-13.5 knots (S/L ~ 2.2).

I appreciate what twistedtree and Psneeld both point out. As someone with a familiarity with physics, a long time boater, and general idiot, I guess in my mind when I think of the "sweet spot" I try to imagine that point where returns start to diminish greatly with increased power. I know that NMPG is not a linear function at any power range, but for most boats isn't there is a point where that curve starts to become significantly steeper? It is that point that I try to find with my boat. Then it is a matter of factoring in other issues like how soon my wife wants to get where we are going.

sine you know physics a little math will be familiar. The prop curve is X=Y^2.5

pick you favorite spot where the curve suddenly starts increasing too rapidly. That will be your sweet spot.

So here is an extremely heavy displacement hull being driven slightly over her "hull speed" of 7.9 knots. We know this because the transom(end of the hull) is ahead of the crest of the stern wave by just a few feet. Note the deep hollow amidships, the depth of this hollow has to do with the heavy displacement of this boat. Her Displacement/length ratio will be around 550. She is pushed beyond hull speed by huge HP, in this case 500+. Fuel burn is of lesser concern than pulling power.

And here we have the other extreme. The 62' x 7'6" Tlingit has a Displacement/Length ratio of about 40. She can cruise at hull speed, 10 knots, with only 30 installed HP. In the towing model (scale speed 10 knots) you can see the bow wave crest is a long way aft, and there's almost no hollow midships or recognizable stern wave. It's pretty easy to understand that this hull, given some more power, might not have a "hull speed" barrier until S/L 1.8 or more......